Voltage references are required to provide a substantially constant output voltage irrespective of changes in input voltage, output current, or temperature. Such references are used in many design applications, such as stable current references, multipliers, control circuits, portable meters, two-terminal references and process controllers, a comparator mode for causing a logic transition, and a servo mode for generating a fixed voltage reference.
Modern voltage references are generally based on either zener diodes or bandgap generated voltages. A disadvantage of conventional bandgap voltage reference circuits is that they comprise resistors of comparatively large value, which resistors should be matched in value with each other. Particularly in integrated circuit (“IC”) processes, in which it is difficult or not possible to fabricate resistors which are accurate and have comparatively high resistance values, said disadvantage is a very significant factor.
The challenge in using CMOS circuit design for generating voltage references is minimizing area for cost reduction. This necessitates the use of purely CMOS components to obsolete area-consuming bipolar devices. Furthermore, in order to further reduce the cost of manufacturing, the circuit design must use standard N-channel and P-channel type CMOS transistors.
In electronic systems including power management systems, it becomes necessary to monitor the supply voltage and effect a decision based upon a comparison of the supply voltage level with respect to a fixed reference voltage. For instance, LDO power-management systems require voltage blocking, referred to as lockout, when the supply is less than the minimum sustainable operating voltage. Also, load-switch and charge-pump systems require a supply voltage-level detection to change the charge-pump voltage levels. Moreover, for system stability and predictable performance, the logic comparison must be predictable and stable over temperature variation. Finally, when these electronic systems are battery powered, minimizing quiescent current becomes paramount. Thus, CMOS becomes the integrated circuit process of choice.
The problem then is to derive a circuit using standard CMOS transistors, N-channel and P-channel type transistors, for monitoring supply voltage with respect to a fixed reference. As a result, it is desirable to provide circuits for this function with a minimum number of transistors and can operate in either of two modes: (1) comparator mode for detecting supply voltage as an independent variable compared to an inherent temperature-independent reference; and (2) servo mode for generating a fixed temperature-independent voltage reference.